The superficial fibers (pars superficialis) surround the upper part of the radius, and are inserted into the lateral edge of the radial tuberosity and the oblique line of the radius, as low down as the insertion of the pronator teres. The upper fibers (pars profunda) of the deeper plane form a sling-like fasciculus, which encircles the neck of the radius above the tuberosity and is attached to the back part of its medial surface; the greater part of this portion of the muscle is inserted into the dorsal and lateral surfaces of the body of the radius, midway between the oblique line and the head of the bone. [2]

The proximal aspect of the superficial head is known as the arcade of Frohse or the supinator arch.

The radial nerve divides into deep and sensory superficial branches just proximal to the supinator muscle — an arrangement that can lead to entrapment and compression of the deep part, potentially resulting in selective paralysis of the muscles served by this nerve (the extensor muscles and the abductor pollicis longus.) [3] Many possible causes are known for this nerve syndrome, known as supinator entrapment syndrome, including compression by various soft-tissued masses surrounding the nerve, and stress caused by repetitive supination and pronation. [4]

Encircling the radius, supinator brings the hand into the supinated position. In contrast to the biceps brachii, it is able to do this in all positions of elbow flexion and extension. [1]

Supinator always acts together with biceps, except when the elbow joint is extended. [6] It is the most active muscle in forearm supination during unresisted supination, while biceps becomes increasingly active with heavy loading. Supination strength decreases by 64% if supinator is disabled by, for example, injury. [7]

The term "supinator" can also refer more generally to a muscle that causes supination of a part of the body. In older texts, the term "supinator longus" was used to refer to the brachioradialis, and "supinator brevis" was used to describe the muscle now known as the supinator.